Ultrasonic imaging systems have long been used in the field of medical diagnostics. Such systems include an ultrasonic transducer, imaging electronics and display apparatus. The imaging electronics actuate the transducer for propagation of incident ultrasonic energy into a nearby patient's body. Within the patient's body, the ultrasonic energy causes echoes at interfaces between body tissues having different acoustical impedance characteristics. Some of these echoes are reflected back to the transducer which converts them to electrical output signals. The imaging electronics process the electrical output signals to cause the display apparatus to produce visual images representing internal structure of the patient's body.
Some ultrasonic systems are capable of imaging in real time. One type of such system employs an ultrasonic probe assembly including a movably mounted ultrasonic transducer enclosed within a chamber containing a fluid ultrasonic couplant material. In one such system, the transducer is mounted for pivotal movement about an axis, and a motor is provided to pivotally oscillate the transducer back and forth about its axis by way of a drive element or shaft coupled between the motor and the transducer. An encoder is coupled to the motor to indicate the instantaneous angular position of the transducer. The imaging circuitry senses both the ultrasonic echo representing output signals, and the encoder signal indicating transducer orientation, to produce an image appropriately corresponding to the spatial pattern of received ultrasonic echoes. The imaging circuitry takes into account both the distance between the transducer and the origin of the echoes, and the transducer angular position, which together defines the location in space of the echoes.
Examples of prior art mechanically scanned transducer ultrasonic imaging systems are set forth in the following U.S. Patents, which are hereby incorporated by reference: U.S. Pat. No. 4,238,962, issued on Dec. 16, 1980, to Taenzer; U.S. Pat. No. 3,886,490, to Green.
In one example of an ultrasonic probe assembly, a stepper motor is used to power the transducer movement while simultaneously providing positional information regarding transducer orientation. The motor is coupled through a gear head comprising one bevel gear which drives another bevel gear, to which the transducer is coupled.
Another probe assembly is powered by a brushless D.C. motor. A rotary transformer is coupled to the motor for use as a position sensor. The transducer is driven by a bevel gear set, one mounted on the motor shaft, the other to the transducer.
Commonly, the ultrasonic transducer and the drive coupling structure, such as the bevel gear arrangements described above, are immersed within a probe housing in a fluid ultrasonic couplant. At least two proposals have been made for effecting a seal of the fluid couplant inside the probe. According to one proposal, the entire probe housing is sealed and its interior is completely flooded with couplant. The quality of the sealing can be very good in this instance, but all the parts within the probe housing, including the electric motor, must work under immersed conditions. This requirement limits the choice of components, and adversely affects reliability of operation.
According to another proposal, rotary or shaft seal is provided within the housing, dividing the housing into a "wet" section and a "dry" section. The transducer and gear coupling arrangement are within the wet section with the motor and encoder apparatus located in the dry section. A drive shaft extends from the motor to the gear coupling apparatus through the rotary or shaft seal. Difficulty has been experienced, however, in effecting complete liquid-impervious sealing by this means. Small leakage of fluid from the wet section to the dry section has been substantially unavoidable. This leakage can result in undesirable wetting of the electric motor components, and/or in the appearance of air bubbles in the couplant within the wet section. The presence of these air bubbles interferes with proper transmission and sensing of ultrasonic energy, and requires the fluid chamber to be refilled whenever such bubbles appear.
Also, neither of these approaches to sealing afford optimum accommodation of changes in fluid pressure in the region of the couplant.
Ultrasonic transducer probe assemblies also include electrical leads which are coupled to the ultrasonic transducer and which extend out of the confines of the transducer probe housing to the imaging circuitry located externally of the housing. Because the transducer itself oscillates, however, it has been necessary to provide for relief of strain in the electrical leads which would arise if the leads were strung taut.
One proposed solution has been to form a service loop in the electrical lead in the vicinity of the transducer. This configuration provides adequate strain protection provided that the lead is properly bent. The skill of the assembler in properly configuring the lead, however, is very important in applying this solution. Often, an inexperienced or inadequaely skilled assembler does not suitably shape the loop to provide adequate strain relief. Additionally, the service loop floats freely in the ultrasonic couplant in the vicinity of the gear drive, and is sometimes caught in the drive, causing malfunction.
It is an object of this invention to provide an acoustic transducer probe assembly having an effective and flexible internal liquid-impervious seal for isolating the wet chamber in which the transducer is located and improved means for providing strain relief in electrical leads coupled to the transducer.